HIGH PERFORMANCE FLIGHT CRYOCOOLER COMPRESSOR
P.B. Bailey and M.W. Dadd
Oxford University, Oxford, UK
N. Hill and C. F. Cheuk
Honeywell Hymatic, Redditch, UK
J. Raab and E. Tward
TRW, Redondo Beach, CA, USA
Published in the proceedings of the Eleventh International Cryocooler Conference
July2000, Colorado
Copyright Kluwer Academic/Plenum Publishers of New York
ABSTRACT
In this paper we report on the development of a next generation flexure bearing compressor which features high efficiency, high capacity per unit mass, enhanced producibility and ease of integration into payloads. The compressor was developed for the 95K High Efficiency Cryocooler programme.
The compressor achieves low mass by using small diameter flexure springs and having a new compact design of magnetic circuit which also has the advantage of being self shielding, thus reducing the radiated magnetic field.
A pair of compressors mounted back to back and driven in anti-phase provides low levels of self-induced vibration, which is further improved by the rigidity of the motor and the characteristics of the new motor and spring designs.
Its ease of integration results from its compact size and the incorporation of a single thermal and mechanical mounting interface in its centreplate. The centreplate incorporates heat spreading both internally for removing compressor heat as well as for spreading the heat to the radiator to which it can be attached.
Producibility has been achieved by transferring the processes developed for manufacturing a similar Oxford designed long life tactical cryocooler.
The compressors are being manufactured by Honeywell Hymatic to a design which has evolved from earlier machines made by Oxford University. TRW will integrate the compressors into the flight qualified 95K High Efficiency Cryocooler which will be delivered to AFRL in October 2000.
INTRODUCTION
A new type of compact linear motor and a new flexure spring design have been developed by Oxford University for linear compressors with the aim of meeting stringent requirements for high efficiency and low mass. The compressors are a key part of the High Efficiency Cryocooler (HEC) which is being developed by TRW.
Prior to this programme 3 single compressors (all with active balancers) and 4 balanced pair compressors were made and assembled at Oxford for TRW, together with a fifth balanced machine assembled by TRW. Two of the balanced pair compressors were delivered by TRW to NASA/JPL for the New Millenium IMAS project. These compressors, which have an identical motor design to the High Efficiency Compressor discussed here, have been subject to extensive thermal, vibration and EMI testing.
The original design has now been taken a step further in a three way collaboration between Oxford, Honeywell Hymatic and TRW, with the aim of making the compressor more rugged, and also introducing a fully controlled assembly and test process more suitable for repeated and consistent quantity production. The assembly and test processes have drawn on the manufacturing and process technology from the Honeywell Hymatic tactical Stirling cooler. In the first half of 2000 two HEC compressors have been built at Honeywell Hymatic, using the new assembly and test processes and have been delivered to TRW.
DESIGN PHILOSOPHY
The key feature of the Oxford design philosophy is the holistic approach to design taken – too many cryocoolers have been designed that are almost impossible to assemble. From the outset, details are incorporated into the design to aid the assembly and testing of the machine. Another important feature is the elimination of many delicate and precise components, which are replaced by simpler parts, combined with extensive use of jigs and fixtures. This approach lends itself to larger production quantities, rather than the 'one-off' approach to earlier builds.
COMPRESSOR DESIGN
The compressor is based on the well-proven 'Oxford' principles of spiral flexure springs and non-contacting clearance seals. The machine is a compact moving piston design, with the piston and cylinder located within the core of the magnetic circuit of a moving coil motor.
The springs are the only component subjected to significant fatigue loading, and these are routinely batch tested at a minimum 25% overstroke to in excess of 108 cycles. The springs have been qualification tested, and the results from this predict a single spring arm reliability of 0.999998 and a reliability for the 96 spring arms used in each compressor of 0.9998.
The linear motor powering the compressor is a new moving-coil design that features a very compact magnet circuit with low flux leakage and consequent high motor efficiency for the size and power. The coil is fully supported on a former, and special attention was paid to maximize the fill factor and increase motor efficiency. The structural integrity of the coil former facilitates transmission of driving forces without relying on the variable strength of the coil potting adhesive, thereby eliminating a common source of compressor failure.
The motor design is self-shielding and features extremely low levels of radiated magnetic field. Test on the similar IMAS compressor showed that the compressor essentially met the requirements of the MIL-STD-461C RE01 test specification measured at 7cm distance.
The design of the compressor is such that it is inherently well balanced with the two 'compressor halves' mounted in line and operated in anti-phase. Tests on the IMAS cooler indicated very low levels of self-induced vibration with 30 W of sine wave input power. The only harmonic above 40mN rms was the second harmonic, and this probably arises from a mismatch in the 'mechanical zero' position between the two compressor halves3.
The two identical compressor halves are mounted on an aluminium alloy 'Centre Plate' that contains all of the cryocooler interfaces. Electrical power is supplied by means of a hermetic feedthrough which is Electron Beam welded to the centreplate. One face of the centre plate provides 45cm2 of thermal interface for heat rejection. A large flange is provided around the connection to the pulse tube for a vacuum tank to be fitted during testing. Gas containment is by means of metal 'O' rings between the end caps and centre plate and an aluminium gasket to seal the fill port. A leakage rate of better than 10-7 mbar litre/sec has been achieved consistently.
Figure 1. Completed compressor.
Particular attention is given to the locking of fasteners and component stacks to preserve the alignment against vibration and redundant locking devices are used to enhance reliability.
The compressor has been designed to operate with a nominal 100 Watts of input power with an additional 50% input power margin. The compressor has an overall length of 226 mm, with end caps 57 mm in diameter and a mass of 2.45 kg.
PRODUCABILITY
Many of the assembly processes involve bonding, and these processes are usually irreversible. Hence the key to quantity production of cryocoolers is the verification (where possible) of each and every stage of the production process, from component manufacture to final assembly.
Material Selection
Materials used in the assembly such as plastics, adhesives and primers are selected from an existing knowledge base of materials with a low out-gassing rate. New materials are extensively tested for their margin of being rendered clean by vacuum bake-out processes. A quadrupole Mass Spectrometer is used extensively for this purpose. Only traceable materials are used in the manufacture of components.
Component Stage
Geometric tolerances commensurate with the functional requirements of the components are specified. During component manufacture, the geometric tolerances form the basis of the method of work holding, while the surface finish requirement defines the manufacturing process. The success of the component manufacturing processes stems from the realisation of the differences between metal and plastic components when they are machined to tight tolerances.
Non-contact measuring methods including laser, optical and air gauging are used extensively to verify the success of the manufacturing processes. Plans for transit and storage protection for each component are designed into the manufacturing processes from the raw materials to the finished products.
Verification and Development of Assembly Processes and Tooling
Assembly processes are verified by testing as defined in process development plans. Sufficient quantities of test pieces are manufactured to render these tests statistically significant. The objective of these tests is to determine the capability of the processes. Data derived from these tests is then used as the standard to which subsequent assembly operations are controlled, rather than the less stringent functional requirement of the assembly itself. If a process is capable of performing to a standard, it is considered to be under control only if it consistently attains the same standard within the natural variation of the process.
The design of the Compressor relies heavily on the use of tooling. The dimensions and geometric tolerances of tooling is inspected and verified before being released for production.
Component Inspection
For the initial builds, 100% inspection of all dimensions is being implemented, but as the component manufacturing processes become fully defined and robust, this will be gradually replaced by 100% inspection of 'critical dimensions only' in future builds.
'Goods Inward' inspection of components in itself is not sufficient. It is a truism that components are at their best immediately after manufacture - from then on it is downhill - every operation, from finishing (deburring/frazing), inspection, cleaning and transport has the potential to damage components. For this reason it is vital with critical components to have a functional inspection of the part immediately prior to assembly. A detailed inspection plan has been compiled specifying the functional inspection requirement of critical parts and the method of inspection, which mimics the assembled conditions of the components. During the initial builds, this inspection procedure will detect errors on parts indicating that the processes need refining. This gateway enables improvements on the quality of components to be made before they are assembled beyond the 'point of no return'.
In-Process Testing
Where practical every stage of the assembly process is verified by some form of in-process testing, both to test the validity of the actual process itself and also to ensure that the process has not had any secondary deleterious effects on the assembly.
Experience has shown that one of the main problem areas is the clearance seal – it is difficult to achieve the correct clearance and easy to lose it. Hence many of the most important tests are those which check the alignment of the assembly and show that there is no friction between piston and cylinder. Placing the friction and alignment tests strategically in the assembly process, the consistency of the free frictionless movement of the finished compressor can be assured.
Test Facility. Many of the tests are carried out on a computer-controlled test rig which also functions as a data logger. Using digital-to-analogue converters the computer controls both DC and AC amplifiers for powering the compressor, together with signal conditioning and data logging functions. Among the functions available is the facility to continuously monitor the drive coil temperature and to shut down any test should this temperature become unacceptably high. This facility is essential in some of the DC tests, which are slow and could easily lead to a coil burn-out if the tests were carried out manually.
Alignment Test. This test verifies the capability of the spring suspension system to effect a linear motion of the piston within the cylinder such that the clearance between them is maintained. The test is carried out immediately after the spring stacks are assembled and aligned. The assembly is then locked to prevent movement, and the test is repeated before the piston is fitted.
Figure 2. Alignment Test – typical result
To perform the test, power is applied to the motor, which is taken through three complete stroke cycles and the run-out error measured. A least-squares polynomial is calculated through the data points; figure 2 shows a 'screen dump' of a typical test result. Note that the sensor used exhibits some backlash, but this is annulled when the polynomial is calculated (visible in the centre of the trace). The repeatability of the measurements is excellent – apart from a certain amount of 'bedding-in' during the first cycle, the three curves follow each other within 0.1m m. The repeatability shown here is an excellent demonstration of the flexure spring suspension system.
The results of this particular test must be treated with a certain amount of caution, as the test is recording not only the linearity of the motion of the cylinder, but also measuring the straightness of the cylinder itself. Thus to make any sense of the test, the 'cylindricity' of the cylinder must be good, and is typically less than 1.5m m. Much of the 'noise' apparent on the trace is repeated on successive test cycles and is due to the surface finish of the cylinder.
From such readings a complete picture of the combined linearity and cylindricity can be built up, and this is then displayed as a three-dimensional plot (figure 3). From this plot the effect of form error and alignment error can easily be separated and the true alignment error evaluated.
Friction Tests. Tests are used to evaluate the friction between piston and cylinder – one for dynamic friction, and one for static friction. The tests are computer-controlled and are performed at several stages throughout the build, both before and after the piston is fitted.
The dynamic test (low frequency sweep) involves driving the compressor through one complete cycle using a 0.01 Hz triangle wave. A curve of current against displacement is plotted and studied for discontinuities and to observe the size of the hysteresis loop.
Process Capabilities. Using the in-process tests outlined above the capability of the alignment and piston assembly processes can be quantified.
The alignment process is expected to produce a mean error of 2.6m m with a standard deviation of 1.67. 99% of the spring suspensions systems that have been aligned by these processes are expected to have an alignment error of less than 6.5m m. The smallest diametrical clearance between the piston and cylinder should then be larger than 13m m for a true frictionless clearance seal design to be realistic.
Figure 3. Alignment test - typical result in 3D plot.
CONCLUSION
A compact, high power and producible compressor design has been achieved. A build procedure has been formulated and line qualified to ensure the consistency of the quality of the compressor. Alignment accuracy of less than 6.5m m (peak-to peak) has been achieved with no evidence of friction in the clearance seal. Innovative in-process test procedures have been designed and are expected to benefit the future manufacture of cryocooler compressors.
ACKNOWLEDGEMENT
We acknowledge the strong support of Thom Davis of AFRL for this project.